Silicone modified solvent borne alkyd resins have been used, usually in minor proportions, for many years in exterior maintenance coatings. Hydroxy functional, low molecular weight silicones such as Z-6018 (Dow Corning) are known to react with various coating resins to form essentially block copolymers having Si--O--C linkages. Such modified resins show improved heat stability and weatherability over unmodified resins. A number of powder coatings (U.S. Pat. No. 4,107,148; Japan Kokai Nos. 73/60748 and 79/152031) comprise mixtures of a polyester and 0.1 to 5 weight percent of a silicone compound. L. M. Parr, (Fed. Soc. Coatings Technology, October 1980), used silicones and functional polymers as modifiers for alkyd resins, especially water reducible silicone alkyds. The fusion processing of Z-6018 with short, medium and long oil alkyds for improvement of air dry maintenance coatings is discussed in the Dow Coring product bulletin (1980). British Pat. No. 1,492,753 relates to a powder coating process utilizing an inorganic pigment coated with an organosiloxane-oxyalkylene copolymer as a flow additive in epoxy thermoset resins. U.S. Pat. No. 3,912,670 teaches radiation curable oils comprising the reaction products of acrylic or methacrylic acid with polyfunctional and cyclic silicones.
The present invention relates to high siloxane content silicon-polyester and silicon-polyester copolymer reins having a silicone content, basis total resin, from about 30-90% and preferably exceeding 40 percent, and which are useful in thermosetting powder coatings.